What is called waterproof zippers and those that are called water resistent are the same thing in my book. The "waterproof" zippers I have used leak over time, making them only water resistent at best.
Installing zippers is a bit more challanging than most simple sewing stitches and may not be a first choice of projects if you are new to sewing. Make sure your help has experience sewing zippers if you decide to do it.
Personally, I wouldn't bother with the zipper if you will be using it with a tarp. As the previous post suggested, just make sure your bivy opening is well protected by the tarp. I used a similar Gore-Tex bivy with a 5 x 5 silnylon tarp for a while. The tarp was just big enough to cover my head and protect my gear and worked well enough.